The explosive growth of the Chinese economy over the past seven years has been linked with a rise in emissions of man-made methane, a study has found.
Methane is the second most important greenhouse gas and, molecule for molecule, it is about 20 times as potent as carbon dioxide in its ability to exacerbate global warming.
Levels of methane in the atmosphere have risen since the Industrial Revolution but in recent years they appeared to have stabilised, leading scientists to believe that the gas may not be so critical in terms of global warming. However, the study found that the decline in methane levels seen during the 1990s was largely the result of the drying out of wetland areas due to climate change, which suppressed a major natural source of the gas.
This, they found, has masked a significant increase in man-made sources of methane from booming Asian economies, particularly China, whose economy has more than doubled since 1999.
"The bad news is that the slowdown in global methane emissions in the past few decades was only temporary - reports of the emissions' control have been greatly exaggerated," said Jos Lelieveld, an atmospheric scientist at the Max Planck Institute for Chemistry in Mainz, Germany.
The study, published in Nature, was carried out by an international team of researchers which included Paul Steele of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO). "Had it not been for this reduction in methane emissions from wetlands, atmospheric levels of methane would most likely have continued rising," Dr Steele said.
"This suggests that if the drying trend is reversed and emissions from wetlands return to normal, atmospheric methane levels may increase again, worsening the problem of climate change," he said.
At present, about two thirds of global methane comes from man-made sources, such as the burning of fossil fuel, the accidental release during drilling for natural gas or from cattle ranching. Levels recorded globally can be affected by natural phenomena such as the El Niño ocean current and volcanic eruptions. They can therefore vary enormously from year to year and from hemisphere to hemisphere.
The largest natural source of methane comes from the bacterial decomposition of organic matter in wetlands. Once in the atmosphere, however, the gas can be quickly broken down.
The scientists tried to tease apart the sources of methane using computer simulations of how the gas is transported in the atmosphere over the past 20 years of atmospheric emissions. They used a method of tracing sources based on the concentrations of a carbon isotope - methane emissions from wetlands, for instance, are substantially depleted in this isotope.
Dr Steele and his colleagues showed that the prolonged drying of many wetland regions in the world - caused by draining and climate change - appears to have resulted in a reduction in the release of methane. They also found that during the 1990s, the release of man-made methane was also stemmed by more efficient methods of extracting natural gas. However, the study indicated how the growth of Asian economies, and resulting rises in fossil fuel use, has led to an upturn in man-made emissions of methane since about 1999.
Paul Fraser, a senior scientist at CSIRO who has been working on the role of greenhouse gases for more than 30 years, said the study was important. "What this work demonstrates is that there have been two opposing processes at work that together have resulted in stable methane emissions and no increase in atmospheric concentration over the past eight years," Dr Fraser said.
"On the one hand, wetlands have been drying up globally and thus emitting less methane. On the other, economic growth in the northern hemisphere, especially in China, has generated increasing amounts of methane from the mining and use of fossil fuels," he said.
"Though it is difficult to predict what will happen to wetlands in the future, it is more than likely that overall methane levels in the atmosphere will increase in the future, due to increasing demands for energy."
Professor Neville Nicholls, of Monash University, in Melbourne, said: "Methane is an important greenhouse gas, and this work is a major step in better understanding why methane emissions are changing. But the relentless increase in carbon dioxide emissions from fossil fuels is, and will continue to be, the most important driver of the global warming that we are witnessing."
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